#### MotoH

##### It's a Capita.
This seems like a very fundamental question, but I have always wondered it. (please move to another section if this is in the wrong place!)

How does a space ship move in space? If there is nothing for a rocket to push against, how does it move?

Any forms of literature would be excellent on this subject, as I am not really sure what to google for information!

Thanks!

MotoH

#### Garth

Gold Member
Stand on a skate board next to a friend on another skate board, have the wheels all pointing in the same direction along the line joining you both. Now push your friend and see what happens.....

Garth

#### MotoH

##### It's a Capita.
So there is an equal and opposite reaction in space? I thought space was a vacuum therefor no molecules to push against in order to go forward?

#### engkatiemarie

Newton's Third Law: For every action there is an equal and opposite reaction.

In space, a vehicle would actually be pushing against the propellant it is ejecting.

#### MotoH

##### It's a Capita.
Oh! So anything can be pushed out of the vehicle, and it will "push" the space craft in the opposite direction.

Very easy to understand now! Thanks!

#### Integral

Staff Emeritus
Gold Member
Better example, stand on the skateboard and throw a brick. (If I did this I would land on my backside, so the assumption is you are a competent skateboarder!)

#### qraal

Oh! So anything can be pushed out of the vehicle, and it will "push" the space craft in the opposite direction.

Very easy to understand now! Thanks!
Unfortunately a lot of people can't accept the fact that throwing stuff out of a rocket very fast is the only way it will move.

#### HallsofIvy

Another way to think about this is the "center of mass". As long as there is no external force, the center of mass of a system remains motionless. if you throw a small mass out of your rocket ship, it is still considered part of the system. In order that the center of mass remain in the same place, the rest of the rocket ship must move in the opposite direction.

Of course, the center of mass of a two-piece system, of greatly different masses, will remain "near" the greater mass. That's why we have throw that small mass (the reaction gasses in the rocket's jet) very, very fast- much faster than we want the rocket to travel.

#### jtbell

Mentor
Unfortunately a lot of people can't accept the fact that throwing stuff out of a rocket very fast is the only way it will move.
Actually, there is another way: throw stuff (from the "outside") at the rocket. That's how a solar sail works. Some people have even suggested using earth-based (or maybe moon-based) lasers to propel a spacecraft equipped with a sail.

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